1. Field of the Invention
The present invention relates to a process for hydrogenation of a conjugated diene polymer, and more particularly to a process for hydrogenation of a conjugated diene polymer using a special catalyst composition.
2. Description of the Prior Art
Generally, polymers obtained by polymerizing or copolymerizing conjugated dienes are widely utilized for commercial purposes. These polymers have residual unsaturated double bonds in their polymer chains. These unsaturated double bonds are advantageously utilized for vulcanization and yet have a disadvantage in that they lack the stability to resist weather conditions, oxidation and ozone due to the presence of a large amount of unsaturated double bonds. Such disadvantage is more severe for block copolymers of conjugated dienes and vinyl aromatic hydrocarbons used as thermoplastic elastomers. Such disadvantage is even more severe when the block copolymers of conjugated dienes and vinyl aromatic hydrocarbons are used as modifiers and transparent impact-resistant materials for styrenic resins and olefinic resins. In the field of exterior materials to which such properties are indispensable, therefore, the block copolymers find limited utility because of this drawback.
This deficiency in stability can be notably improved by hydrogenating such conjugated diene polymers and consequently eliminating the unsaturated double bonds persisting in the polymer chain thereof. Numerous methods have been so far proposed for hydrogenating conjugated diene polymers in the presence of suitable and effective hydrogenation catalysts. Typical hydrogenation catalysts can be classified into two types:
(1) heterogeneous catalysts having compounds of nickel, platinum, or palladium deposited on supports such as active carbon, silica, alumina, or calcium carbonate;
(2) homogeneous catalysts, for example, Ziegler-type catalysts obtained by causing an organic acid salt of nickel, cobalt, iron, or chromium to react with a reducing agent such as an organic aluminum compound, and organometallic compounds such as Ru, Rh, Ti, or La compound.
Although widely used in industry, heterogeneous catalysts have lower activity than the homogeneous catalyst. Therefore, it requires a large amount of heterogeneous catalyst and the hydrogenation must be conducted at higher temperature and pressure. In contrast, less homogeneous catalyst is needed and the hydrogenation can be conducted at mild temperature and pressure.
Hydrogenation using a heterogeneous catalyst is described as follows. First, the polymer to be hydrogenated is dissolved in a suitable solvent. Then, the polymer is brought into contact with hydrogen in the presence of a heterogeneous catalyst. When the polymer is hydrogenated, contact between the polymer and the catalyst is difficult because of the influence of the viscosity of the reaction system and the influence of stereohindrance of the polymer. Moreover, the hydrogenation requires higher temperature and pressure, and hence, decomposition of the polymer and the gelation of the reaction system tend to occur. Also, in the hydrogenation of a copolymer of a conjugated diene with a vinyl aromatic hydrocarbon, even hydrogenation of the aromatic ring portion takes place due to high temperature and pressure and it has been difficult to selectively hydrogenate only the double bonds in the conjugated diene portion. In addition, since the polymer is strongly adsorbed on the heterogeneous catalyst, it is impossible to completely remove the catalyst from the hydrogenated polymer solution.
On the other hand, with the homogeneous catalyst, the hydrogenation proceeds normally in a homogeneous system. Therefore, compared with the heterogeneous catalyst, the homogeneous catalyst is generally high in activity and a small amount of catalyst enables a satisfactory hydrogenation to be effected at low temperature and low pressure. In addition, when appropriate hydrogenation conditions are selected, it is possible to preferentially hydrogenate the conjugated diene portion of a copolymer of a conjugated diene with a vinyl aromatic hydrocarbon, while not hydrogenating the aromatic ring portion.
Conventional processes for hydrogenation of a conjugated diene polymer using a homogeneous catalyst are summarized below.
U.S. Pat. No. 4,980,421 discloses a process for hydrogenating a conjugated diene polymer using a hydrogenation catalyst including a bis(cyclopentadienyl)titanium(+4) compound, an alkoxy lithium compound, and an organometallic compound (such as aluminum, zinc, or magnesium compound). This hydrogenation catalyst has high activity and can be used under mild conditions.
U.S. Pat. No. 5,270,274 discloses a hydrogenation catalyst composition including a bis(cyclopentadienyl)titanium(+4) compound, a polarized carbonyl group or epoxy group-containing compound, and an organic lithium compound. The unsaturated double bonds in the conjugated diene polymer can be preferentially hydrogenated. The hydrogenated polymer has superior physical properties and weather resistance.
U.S. Pat. No. 5,244,980 discloses a hydrogenation process including terminating a living conjugated diene polymer with hydrogen, and then adding an organo alkali metal and a Tebbe's catalyst.
U.S. Pat. No. 5,886,108 discloses hydrogenating a living conjugated diene polymer using a Tebbe's catalyst prepared by the reaction of a bis(cyclopentadienyl)titanium(+4) compound and trimethyl aluminum.
U.S. Pat. No. 5,985,995 discloses a process for producing a hydrogenated rubber, which allows an easy and effective deactivation of the living polymer before the hydrogenation step. The deactivation of living polymer is carried out using alkyl silicon halide or alkyl tin halide. The catalyst used in the hydrogenation of the deactivated polymer can be bis(cyclopentadienyl)Ti(PhOCH3)2.
U.S. Pat. No. 5,948,869 discloses a catalyst composition effective in the selective hydrogenation of conjugated diene polymer, which includes a bis(cyclopentadienyl) titanium(+4) compound, an alkyl zinc or alkyl magnesium, and a modifier of ethers or aromatic hydrocarbon compounds.
European Patent No. 0434469 A2 discloses a catalyst composition for hydrogenating a conjugated diene polymer, which includes a bis(cyclopentadienyl)titanium compound, an organoaluminum compound or organomagesium compound, and a polar compound of ether or ketone.
European Patent No. 0544304A discloses a catalyst composition including a bis(cyclopentadienyl) transition metal compound, a polarized compound of carbonyl group-containing compound or epoxy group-containing compound, an organic lithium compound, and a reducing organometal compound such as aluminum compound, zinc compound, or magnesium compound, for example, triethyl aluminum.
Although various catalyst compositions have been developed to hydrogenate the conjugated diene polymer, there is still a need to develop a new catalyst composition for hydrogenation of conjugated diene polymers.